A problem facing the designer of a high pressure metal halide lamp intended for operation at low power (150 W and below) is that of achieving adequate partial pressure of metal halides, for example iodides, chlorides and bromides of tin, sodium indium and thallium, in the lamp fill. This generally requires the lamp to be arranged to operate at a relatively high minimum arc tube temperature. Conventionally, metal halide lamps have fused silica envelopes, but arc tubes made of a light-transmitting crystalline ceramic material, for example polycrystalline alumina, sapphire or a spinel, allow lamps to operate at cool spot temperatures above 600.degree. C., and particularly between 700.degree. C. and 1000.degree. C.
EP55049 (THORN EMI) discloses a sealing composition including MgO, Al.sub.2 O.sub.3 and SiO.sub.2. The preferred compositions contain up to 5% by weight of nucleating agent, from 51 to 62% by weight of silica, from 13 to 21% by weight of alumina and from 25 to 28% by weight of magnesia. Glassy seals produced, according to this patent, tend to soften at about 700.degree. C. and, in general, would be unsatisfactory for use in lamps operating with end temperatures in the range of 700.degree. to 1000.degree. C., for example, because the fill would be able to escape through the softened and so weakened lamp seal.
EP55049 specifically discloses a sealing composition (referred to hereinafter as Xo) consisting of 28% by weight MgO, 20% by weight Al.sub.2 O.sub.3 and 52% by weight SiO.sub.2, which has been widely used as a sealing material for crystalline Ceramic Metal Halide (CMH) lamps. This composition has the advantage of melting, and so being sufficiently fluid to flow and penetrate between the parts to be joined, at a temperature, typically around 1385.degree. C. (as measured by Differential Thermal Analysis (DTA)), which is low enough to allow a preferred, so-called multihead sealing technique to be adopted. A disadvantage of this known sealing composition, however, is that it produces a glassy seal which softens and so is weakened at a temperature around 700.degree.-800.degree. C. which is generally lower than the end operating temperature of CMH lamps. Due to this softening at relatively low temperatures, a recrystallisation treatment in the form of a separate thermal crystallisation schedule is required on virgin lamps to deliberately develop crystalline phases such as .mu.- and .alpha.-Cordierite. The presence of these phases raises the softening temperature in the mixed glassy and crystalline seal. However, during lamp operation with end temperatures above 850.degree. C. crystallisation of the seal tends to continue to completion. The major phase present (.mu.-Cordierite) is metastable and transforms to .alpha.-Cordierite which has a low coefficient of thermal expansion (TE=25.times.10.sup.-7 /.degree.C.) as compared with that of the crystalline ceramic arc tube. At higher temperatures the .alpha.-Cristobalite phase is formed which suffers from a phase transformation on cooling. Volume changes occur during these transformations and due to differential contractions or expansions of the phases present when cooling or heating the arc tube. The overall effect usually increases the stress in the seal area resulting in cracking of the alumina monolith and subsequent lamp failure during switching. Hitherto, this property of the sealing material has limited lamp design to operation at end temperatures below 850.degree. C.
GB 1544779 (National Research Development Corporation) describes a glass ceramic composition comprising MgO 20-35 mol %, Al.sub.2 O.sub.3 20-35 mol %, SiO.sub.2 30-60 mol %, TiO.sub.2 7-20 mol % and ZnO 0-7 mol % but this has the major disadvantage of having a much higher melting temperature (1450.degree. C. as measured by DTA) than that of the afore-mentioned composition Xo.